With increasing use of optical communications systems, it has become increasingly important to provide for appropriate testing of optical fibers in fiber cables of such systems. A fiber cable of an optical communications system may comprise a large number of, for example 48, individual optical fibers, and may extend over a large distance of many kilometers between its ends. It is desirable to be able to measure characteristics of many kilometers between its ends. It is desirable to be able to measure characteristics of each fiber in such a cable, in particular the attenuation of optical signals at two or more different optical signal wavelengths, such as 1300 and 1550 nm, which are typically used in optical communications systems. The attenuation measurements can desirably include measurement of attenuation of optical signals in each direction at each wavelength on each fiber, and total return loss of an optical fiber path which may also include optical fiber connectors and splices which give rise to increased losses.
In order to carry out such measurements, it is known to provide two test units which are employed, one at each end of a fiber cable, to test the fibers by transmitting an optical signal via each fiber between the units, in each case determining the optical signal attenuation at the receiving end. Such an arrangement requires that the units be calibrated with respect to one another. For such calibration, typically the units are brought together and optically coupled via a jumper, and the receiving unit stores for each wavelength a reference power level of the received optical signal, which stored reference is used for determining attenuation during subsequent testing. However, this has disadvantages in that the jumper must then remain connected for accurate testing, making it difficult to test fibers with different connectors, and there is a risk of the stored references being erased through operator error. In consequence, recalibration is frequently necessary, requiring that the units again be brought together as described above.
In Higginbotham et al. U.S. Pat. No. 4,234,253 issued Nov. 18, 1980 and entitled "Attenuation Measuring System" there is described a fiber optic attenuation measuring arrangement in which a feedback loop is used in a transmitter to maintain a constant output power level of a transmitted optical signal, which includes a test signal together with a higher-amplitude timing pulse. At a receiver, the timing pulse is separated and used to demoduate the test signal, which is compared with a reference signal to determine attenuation of a fiber under test. This reference is not concerned with measuring attenuation at different optical signal wavelengths.
In Heckmann U.S. Pat. No. 4,673,291 issued Jun. 16, 1987 and entitled "Method Of And Device For Measuring The Attenuation In Optical Waveguides" there is described an optical attenuation measuring arrangement in which the light power of an optical signal input to a fiber is encoded on the signal using pulse frequency modulation, and this is demodulated at the receiver to be used in determining attenuation of the optical signal by the fiber. This reference also is not concerned with measuring attenuation at different optical signal wavelengths.
In Maslaney et al. U.S. Pat. No. 4,726,676 issued Feb. 23, 1988 and entitled "Optical Signal Power Measurement Method And Apparatus" there is described an optical attenuation measuring arrangement in which optical test signals of different wavelengths are modulated with respective AC signals to identify the respective wavelengths to a receiver. A comparison value, which takes into account the wavelength-dependent sensitivity of a detector of the receiver, is stored in the receiver for each optical signal wavelength and is used with a received optical signal power level to determine attenuation of the optical signal transmitted via an optical fiber. This arrangement assumes a constant power level of the transmitted optical signal, and requires as many different modulating AC signal frequencies as there are optical signal wavelengths.
While these known arrangements provide various improvements over the testing arrangement initially described above, there remains a need to facilitate attenuation measurement of optical fibers at different wavelengths in a manner which is convenient and is not prone to operator error.
An object of this invention, therefore, is to provide an improved method of and measuring attenuation of an optical fiber, and improved apparatus for use in carrying out this method.